In this proposal several series of potent inhibitors of the enzyme methionyl-tRNA synthetase from the protozoan parasite Trypanosoma brucei will be optimized with long term goals to arrive at new therapeutics for treating human African trypanosomiasis. The research will be carried out by a highly experienced team formed by scientists at the University of Washington and PATH with expertise covering: structure-based drug design, medicinal chemistry, parasitology, pharmacology, toxicology, and preclinical testing. The proposed research is based on the strong progress made during the current funding period where multiple series of T. brucei methionyl-tRNA synthetase inhibitors were obtained that showed nanomolar potency against parasite cells, low toxicity to mammalian cells, oral and CNS availability, and efficacy in murine infection models. The proposed work will have four specific aims. Pre-defined criteria will be used to pass or fail compounds coming to each biological test, and the results will continually be fed back into the iterative design process. Th potential for drug resistance will also be examined. The goal for this project is to identify one preclinical candidate and one to two backup compounds that are ready for comprehensive GLP preclinical pharmacology and toxicology studies for further development.